Conventional cellular mobile radio telephone systems are controlled by at least one mobile switching center, or MSC (also known as a mobile telephone switching office), at least one base station, and at least one mobile station. The mobile switching center constitutes an interface between the radio-based cellular system and the public switching telephone network. The base station acts essentially as a conduit for information between the mobile stations and the mobile switching center. Calls to and from mobile subscribers are switched by the mobile switching center. The mobile switching center also provides all signalling functions needed to establish the calls.
In order to obtain adequate radio coverage of a geographical area, plural base stations are normally required. The area is divided into cells, and each cell may either be serviced by its own base station or may share a base station with a number of other cells. Each cell has an associated control channel over which control (non-voice) information is communicated between the mobile stations in that cell and the base station. Generally, the control channel includes a dedicated channel at a known frequency over which certain information is communicated from the base station to mobile stations, a paging channel for unidirectional transmissions of information from the base station to the mobile stations, and an access channel for bi-directional communications between the mobile stations and the base station. These various channels may share the same frequency, or they may operate at different respective frequencies.
In addition to a single control channel, each cell is assigned a predetermined number of voice channels for communicating the content of a communication between two subscribers. That content may be analog or digitized voice signals or digital data signals. Depending on the access mode of the cellular system, each voice channel may correspond to a separate frequency in Frequency Division Multiple Access (FDMA), a separate frequency and time slot or slots in Time Division Multiple Access (TDMA), or a separate code in Code Division Multiple Access (CDMA). The present invention may be implemented using any of these multiple access techniques.
Typically the cells are of relatively small size, and there is a high probability that a mobile station will move from one cell to another. The process of transferring an ongoing mobile communication from one cell base station to another cell base station is called "handoff". It is important that handoffs be accomplished rapidly and reliably if communications are to continue without interruption as mobile stations exit one cell and enter another.
The mobile switching center supervises the handoff of mobile station call connections between various base stations. Each base station continuously measures a signal parameter, typically signal strength, of ongoing calls of mobile stations within its cell. When the measured signal parameter associated with a particular mobile station falls below a predetermined threshold, indicating that the quality or strength of that call connection is or is becoming unacceptable, the base station requests a handoff of that call connection from the mobile switching center. The mobile switching center selects a target base station receiving the strongest signal from the mobile station to be handed off and an associated voice channel to take over the handedoff call connection. In other embodiments, handoff decisions may be based not only on signal strength but also on other factors such as signal quality based on bit error rate (BER), hysteresis, etc. or a combination of factors. The original base station orders the mobile station to tune to the selected voice channel of the target base station.
Unfortunately, each cell is allocated only a limited number of voice channels. During peak use periods or in congested, urban areas, many of the available voice channels may be occupied. In those situations where a target base station has no available voice channels, a mobile station cannot be handed off, and a new target base station must be selected. Selection of a second target base station often is either not possible or unacceptable. First, there may not be any other target base stations receiving a signal of sufficient strength from the mobile station to be handed off. Second, by the time a base station having an available voice channel is located, the call connection may have deteriorated to an unacceptable level or been lost altogether. Third, like the original target base station, subsequent target base stations may not have available voice channels. Handoff requests for ongoing call connections are not the only requests for available voice channels in a particular cell. New call connection requests also compete for available voice channels.
One system which addresses the problem of blocked handoff requests is disclosed in U.S. patent application Ser. No. 07/341,584, filed Apr. 21, 1989, entitled "A Method of Reducing Blockages in Handing Over Calls in a Mobile Cellular Telephone System," which is incorporated herein by reference.
U.S. Pat. No. 4,670,899 to Brody et al. discloses a dynamic load distribution system where ongoing calls are selectively transferred to adjacent cells in accordance with the traffic level in order to reserve channels for handoffs and new calls. In one mode of operation called "directed retry", voice channels are reserved for incoming handoffs by directing a particular cell to deny access to mobile transceivers initiating new calls. Cells in a directed retry state refuse acceptance of new calls in order to reserve channels for handoff attempts. This directed retry state is based on the voice channel occupancy level of a cell. If the number of handoff requests exceeds the number of available channels, reserved and unreserved, a new call or directed retry request is not accepted.
While the Brody system makes some provision for call handoffs, Brody's focus is at the multiple cell level. Brody is primarily concerned with balancing the call connection load among all of the cells. Thus, handoff requests to a particular target cell are not considered in the load balancing process.
It would be desirable to prioritize handoff requests for individual target base stations. In addition, it would be desirable to set up storage queues of handoff requests for each target base station so that when a voice channel becomes available at a particular, target base station, a queued handoff request to that target station is serviced before other handoff and call requests.